1. Field of the Invention
The present invention relates to an illumination apparatus which illuminates a portion to be illuminated with light from light emitting members such as a light-emitting diode (LED), and an image projection apparatus in which the illumination apparatus is used.
2. Description of the Related Art
Examples of a condenser illumination apparatus which illuminates a specified portion with high efficiency have heretofore been known such as a car headlight, stand illumination, spot light, flashlight, and illumination unit for a data projector. In this condenser illumination apparatus, light from a light emitting source closer to a point source is reflected by a reflection unit whose reflection shape is devised, directivity of a light flux of the reflected light is enhanced by an optical lens, and the reflected light is usually used to effectively perform condenser illumination.
In the same manner as in conventional illumination, even in these condenser illumination apparatus, there is much demand for obtaining a brighter illumination light without excessively enlarging a size of the apparatus. In order to obtain the brighter illumination light, the size of the condenser illumination apparatus tends to increase. Particularly to enlarge an applied power of the light emitting source, thereby to increase q quantity of output light, and additionally to enhance a condenser capability, the reflection unit or optical lens is applied which is relatively enlarged with respect to the light emitting source. Therefore, to obtain brightness with good condenser efficiency, the size of the illumination apparatus has to be necessarily enlarged with respect to the light emitting source. In other words, with a small-sized light emitting source which has a high output and which is close to the point source, it is also possible to miniaturize the whole illumination apparatus. From this demand, the miniaturization of the light emitting source of a conventional system has also been advanced, and particularly a small-sized light emitting source by a discharge type from which the high output is possible has been effective means at present. Additionally, even with the light emitting source of the small-sized discharge type, the driving by a high-voltage power source is required in which it is difficult to reduce a circuit scale. There are other many problems with respect to the miniaturization of the total illumination apparatus. It is said that the miniaturization has already substantially approached limitation.
On the other hand, LED is remarkably noted as a next-generation small-sized light emitting source nowadays. The LED has heretofore had advantages such as small size, high durability, and long life, but has been mainly used as indicator illumination for various instruments or a confirmation lamp of a control stage because of restrictions of emission efficiency and emission output. However, in recent years, the emission efficiency has been rapidly improved, and it is said to be a matter of time before the emission efficiency exceeds that of a high-pressure mercury lamp or fluorescent lamp of the discharge type assumed to have heretofore had highest efficiency. By appearance of the high-efficiency high-brightness LED, the high-output light emitting source by the LED has rapidly been brought into a practical use. In recent years, in addition to red and green, a blue LED has entered a practical-use stage, and this also accelerates the application of the light emitting source. In actual, a plurality of high-efficiency high-brightness LED are used to start the practical use in traffic lights, large-sized full-color displays for outdoors, various car lamps, and backlights of liquid crystal displays in the cellular phones, which has heretofore been impossible in brightness or efficiency.
It is thought that this high-efficiency high-brightness LED is also applied as a promising small-sized light emitting source of the illumination apparatus requiring a condensing property. The LED is originally superior to the other light emitting sources in life, durability, lighting-on speed, and simplicity of a lighting-on/driving circuit. Above all, the blue color is added, three primary colors are obtained as spontaneous-light emitting sources, and an application range of a full-color image display apparatus has therefore been enlarged. Typical examples of the illumination apparatus whose condensing property is demanded include a projector display apparatus (image projection apparatus) in which a display image is formed and projected from image data. The image projection apparatus has heretofore separated desired primary colors from a white-based light emitting source by color filters, and has subjected the image data corresponding to each color to spatial light modulation. When the light obtained by the spatial light modulation is spatially or temporally synthesized, color image display is possible. When the white-based light emitting source is used, only the desired color is separated and used. Therefore, the colors other than the separated color are uselessly discarded by the filter in many cases. In this respect, since the LED emits the light of the desired color itself, a necessary quantity of light can be emitted when necessary. As compared with the conventional white-based light emitting source, the light is not wasted, and the light of the light emitting source can be used with good efficiency.
This superior application condition of the LED has been noticed. For example, Jpn. Pat. Appln. KOKAI Publication No. 11-32278, U.S. Pat. No. 6,227,669B1, U.S. Pat. No. 6,318,863, and the like disclose an example in which the LED is applied to the illumination apparatus for the image projection apparatus. The technique disclosed in these publications comprises: disposing a plurality of LEDs to secure a quantity of light; condensing some of fluxes from the individual light emitting sources by optical elements such as the optical lens; and controlling the fluxes so that an image display element as a modulation device to be irradiated is well defined at an allowed incidence angle. For the image display elements such as a liquid crystal device broadly used in general, since the allowed incidence angle is small, it is supposedly ideal to form the flux having higher parallelism and to irradiate the elements. This is a very important point in enhancing light use efficiency in the image display element.
Moreover, as the characteristic of the LED, it is generally known that heat is generated with the emission of the LED and that the emission output of the LED drops in inverse proportion to the increase of the generated heat. To solve the problem, in Jpn. Pat. Appln. KOKAI Publication No. 6-13652, it is disclosed that the current is controlled in order to prevent the heat generation, and driving (pulse driving) is performed in order to obtain a non-emission time. Accordingly, the quantity of emitted light is maintained.